Researchers have devoted considerable effort to developing processes for converting boehmite to alpha alumina. Boehmite is a metastable oxyhydride which is readily obtained by the partial dehydration of gibbsite or alumina gel, the latter being a pseudoboehmite. The dehydration of boehmite proceeds through a series of transition aluminas before finally being converted to alpha alumina.
In contrast with the stepwise conversion of boehmite to alpha alumina at temperatures above 1000.degree. C., diaspore converts directly to alpha alumina at low temperatures (about 400-500.degree. C.). This low temperature conversion is accomplished by a topotactic transition from diaspore to corundum which eliminates formation of the transitional aluminas. Deflandre in 1932 was first to report that diaspore is transformed to corundum by heating in air to a temperature of about 450-600.degree. C. However, natural deposits of diaspore are relatively scarce, and the natural material is impure.
We have found that when substantially pure diaspore is heated to an elevated temperature in a vacuum, a new transition alumina is formed. Further heating of the transition alumina in air produces alpha alumina having increased surface area compared with alpha alumina formed directly by heating diaspore in air.
Processes for producing high surface area alpha aluminas are known in the prior art. For example, Siegel et al U.S. Pat. No. 5,128,081 discloses a method of producing aluminum oxide nanocrystalline ceramic material. The material is formed by oxidizing condensed aluminum powder at an elevated temperature of about 1000.degree. C.
Mitchell U.S. Pat. No. 4,012,337 discloses a process for making high surface area alpha aluminas. The starting material is Missouri grey diaspore, a relatively impure form of diaspore (beta alumina monohydrate). The diaspore is heat treated at temperatures ranging from about 300.degree. to 600.degree. C. while maintaining the ambient water vapor pressure at less than about 10.sup.-1 torr, and preferably about 10.sup.-1 torr or lower.
Mitchell reported producing alpha alumina having a surface area as high as 168 m.sup.2 /g. The x-ray diffraction pattern reported in Example 1 of U.S. Pat. No. 4,012,337 is consistent with an alpha alumina product.
As used herein, the term "alumina hydrate" refers to hydrated alumina oxide, or Al.sub.2 O.sub.3 .multidot.xH.sub.2 O wherein x varies between about 0.5 and 3. The term "diaspore" refers to beta alumina monohydrate, or Al.sub.2 O.sub.3 .multidot.H.sub.2 O. The term "substantially pure diaspore" refers to diaspore having a purity of at least about 90 wt. %.
A principal objective of the present invention is to provide a new transition alumina formed in the transition from diaspore to alpha alumina.
A related objective of the invention is to provide a method for making the new transition alumina.
A further objective of the present invention is to provide a method for making alpha alumina having high purity and increased surface area.
Additional objectives and advantages of the present invention will occur to persons skilled in the art from the following detailed description of our invention.